1. FIELD OF THE INVENTION
The present invention relates to a stabilized polycarbonate resin composition, in particular a polycarbonate resin composition stabilized by the addition of a specific organic cyclic phosphite.
2. DESCRIPTION OF THE PRIOR ART
Polycarbonate resins exhibit high strength and high stiffness and have excellent frictional wear resistance. Accordingly, polycarbonate resins are widely used in the production of automobiles parts, such as bumpers, and parts for various precision machinery such as business machines.
Further, polycarbonate resins have excellent transparency and color tone and are, thus, used in optical fields including laser disks or in the production of automotive mirrors polycarbonates are also useful particularly as engineering plastics.
However, the molding of a polycarbonate resin must be generally conducted at a temperature as high as about 350.degree. C. At this high temperature, the carbonate linkages of the backbone chain are broken down during molding, thus, causing discoloration and/or lowering of the mechanical strength of the polycarbonate resin. Further, in molding a polycarbonate resin into an article having a thin wall or a complicated shape, a particularly high temperature is necessitated in order to lower the melt viscosity of the resin. Again, the thermal deterioration of the resin is significant.
Thus, the use of various antioxidants has been proposed. However, the discoloration of a polycarbonate resin is not inhibited by the use of a phenolic antioxidant. Stabilized polycarbonate resin compositions which have been proposed up to this time include, for example, a composition comprising a polycarbonate resin having an average molecular weight of 15000 to 18000 and a phosphite ester (see Japanese Patent Laid-Open No. 126119/1983), a polycarbonate resin composition containing a fatty acid ester of a polyhydric alcohol, a phosphate ester and a phosphite ester (see Japanese Patent Laid-Open No. 287954/1986), a polycarbonate resin composition containing a pentaerythritol phosphite (see Japanese Patent Laid-Open No. 112655/1988) and a polycarbonate resin composition containing a phosphonium sulfonate and a phosphite ester (see Japanese Patent Laid-Open No. 14267/1989). Such compositions are also unsatisfactory. Accordingly, there is a need to develop a polycarbonate resin composition which is sufficiently resistant to thermal deterioration in high-temperature molding.